The present invention relates to an aqueous ink, and more particularly to an aqueous ink comprising a polymeric dyestuff which is formed by binding a polymer including amino groups with a particular dye, with the addition of a water-soluble viscosity-adjustment agent thereto when necessary.
In general, aqueous inks have a variety of advantages over oil-base inks. For instance, when an aqueous ink is used in writing instruments, it seldom spreads or penetrates into substrates (for example, paper) to the back side thereof, and is free from unpleasant odors and toxicity. More particularly, when an aqueous ink is employed in a ball-point pen, it is advantageous over oil-base inks in that it is free from globbing and skipping during writing, yielding a better quality written image than in the case of oil-base inks. Therefore, aqueous inks are used not only for such writing instruments, but also in a variety of writing and recording apparatuses, for example, as a jet ink in an ink-jet printing apparatus.
On the other hand, when oil-base inks are employed in writing instruments, they have certain advantages over aqueous inks in that the written ink images, once dried, are more resistant to water than aqueous ink images. In addition, written ink images of an oil-base ink containing a solvent with a low boiling point dry much more quickly than aqueous inks and are less subject to smearing by physical pressure than aqueous ink images.
Aqueous inks of the so-called solution type, in which a dye is dissolved in water, are generally used. When an aqueous ink of this type is employed for writing on a sheet of paper, and if the lines or images written by that ink happen to come into contact with water or something wet or damp, the lines or images immediately spread and become blurred due to the poor water-resistance thereof. This is a shortcoming of aqueous inks of the solution type.
Within this category of aqueous inks, there is an aqueous ink in which a basic dye is contained in order to yield clear written or printed images with high density. This type of aqueous ink, however, is not resistant to light (i.e., fades easily).
Further, since, with aqueous inks of the solution type, a dye is dissolved in water but the dye cannot be dissolved beyond a certain concentration, there is a limit to the written or printed image density that can be obtained by use of this aqueous ink, unless the amount of the ink applied to the sheet of paper per unit area of written or printed images is increased. However, if the amount of the ink applied to the paper per unit area of the written or printed images is increased, the written or printed images do not dry quickly, and it may occur that those written or printed images spread before drying.
In order to improve upon aqueous inks with respect to the above-described shortcomings, a variety of methods have been proposed. In one method, a pigment is mixed with an emulsion of a polymeric material when preparing the aqueous ink. In another method, a polymeric dyestuff, which is prepared by bonding a water-soluble dye with a water-soluble resin, is dissolved in water when preparing the aqueous ink.
The former method, however, has the shortcomings that it does not provide sufficient dispersion stability of the pigment for practical use, since the pigment employed has a comparatively great specific weight. When the pigment is used in this method, the use of the pigment alone does not provide a clear color; but, if a dye is added to the pigment in order to obtain a more clear color, the written or printed ink images become considerably vulnerable to water.
The latter method does not provide written or printed images with sufficient water-resistance for practical use.
When an aqueous ink of any of the above-described conventional types is employed in ball-point pens, it is necessary, due to the low viscosity of the aqueous ink, to employ a ball-point pen with a relatively complicated structure as shown in FIG. 1. Such a ball-point pen comprises an ink magazine member 2 for holding a porous material 1 therein, and a writing point 5 for holding a ball 4 and an ink-leading core member 3 disposed adjacent to the ball 4.
If conventional aqueous ink is placed in a ball-point pen intended for use with an oil-base ink, as shown in FIG. 2, comprising a writing point 5 for holding a ball 4, and an ink magazine 2, with a much simpler structure than that of the ball-point pen for use with an aqueous ink, and if the writing point 5 is directed downwards, the aqueous ink flows down from the writing point 5, due to the low viscosity thereof, by the pressure applied to the ink under its own weight within the ink magazine 2. In other words, the conventional aqueous ink cannot be employed in the ball-point pen having such a simple structure, in which the aqueous ink is directly placed in the ink magazine 2, as shown in FIG. 2.